The two-stroke engine, also referred to as the two-cycle engine, has long been the power plant of choice for applications where power to weight ratio and mechanical simplicity are critical parameters for the operator. This is evident by their wide spread use as outboard motors, motorcross motorcycle racing engines and as the power plants for small, hand held tools such as chain saws and weed cutters. Although the large power to weight ratio of these engines is a desirable characteristic for automobile power plants, their high unburned hydrocarbon emissions (from short circuited air fuel mixture during the scavenging process) and the attendant fuel economy penalty has precluded their widespread acceptance into these markets.
Typical in these engines is a simple exhaust gas scavenging system established mainly by ports in the cylinder head that are covered and uncovered by movement of the piston. Thus, numerous complicated and expensive seals, valves and related components required in four cycle engines are omitted and not required.
As the CAFE standards for the automobile fleets have increased, the industry has placed even more of a premium on the power to weight ratio of the engine. A small engine of the same power as a larger one lowers the weight of the vehicle and enables designs of smaller frontal area (less wind resistance). Both of these design factors have beneficial effects on fuel economy.
Interest in two-stroke engines is very high in the automotive industry yet the problems of unburned hydrocarbon emissions remains unsolved. Also, legislation on exhaust emission for off-highway vehicles, lawn and garden equipment and marine craft has brought the emission problems of the two-stroke engine to the forefront of those industries. The industries, both recreational and automotive, are anxious for an economical way to control the emissions, in particular the unburned hydrocarbon emissions, and improve the fuel efficiency from two-stroke engines.
Numerous U.S. patents and other publications discuss the operation and characteristics of these engines, examples including U.S. Pat. Nos. 4,995,354; 4,960,097; 4,936,277; 4,903,648; 4,556,030; 4,576,126; 4,399,778; and a description on pages 9-78 through 9-114 from Marks' Standard Handbook for Mechanical Engineers, Eighth Edition published by McGraw-Hill Book Company, 1978; and pages 299 through 356 of Chapter 7 of The Basic Design of Two-Stroke Engines by Gordon P. Blair, published by The Society of Automotive Engineers, Inc., 1990, all of these references including the complete text of the latter reference being incorporated by reference into this specification. In Marks', for example, on page 9-111 it is stated "in carbureted engines where intake pressure exceeds exhaust (as in two-cycle engines) raw-mixture loss to the exhaust during the valve-overlap period creates very high hydrocarbon emissions. Emissions from two-cycle carbureted engines may be 10 times higher than four-cycle engine emissions."
The massive quantity of unburned hydrocarbons discharged by the exhaust contribute greatly to inefficiency, waste of fuel, and to pollution of the atmosphere, all of these problems being matters of great concern at all levels of society including individual, manufacturer, governmental and international. To some extent these problems have been ignored by continuing the old technology or by choosing alternative power sources with their own inherent disadvantages such as higher cost, higher complexity and lower power-to-weight ratio.
In addressing the above-mentioned problems and operational characteristics in two-cycle engines engineers and mechanics have dealt with a variety of structural components, seeking improvements and solutions. Typical carburetor and throttle devices vary the air/fuel ratio or the rate or directional path of air/fuel flow, or timing, ignition, fuel composition, etc.
A principal focus herein is the high degree of unburned hydrocarbons in the exhaust gas of two-cycle engines due to short circuiting of fuel in the scavenging process. Typically, the carburetor is adjusted to a selected air/fuel ratio, and then the flow of this mixture is throttled by an appropriate valve. In an outboard two-cycle engine the up-stroke of the piston creates a suction which draws in the mixture the flow of which being throttled by partial blockage of flow into the crankcase.
One alternative control technique used in an engine under the commercial name Orbital, is to use fuel injection directly into the cylinder. Inlet air is pumped into the cylinder to scavenge or clean out exhaust gas. Later, as the piston rises and closes the inlet air port, fuel injection follows. In theory this should substantially eliminate unburned fuel from short circuiting since the scavenging air passing through the cylinder head is not carrying the new charge of fuel with it. On the negative side is the added work input of high pressure fuel injection directly into a closed cylinder head, as compared to the Roots blower low pressure air flow (1 to 11/2 atmospheres) which carries the fuel into the cylinder via a typical simple and inexpensive carburetor. The air/fuel mixture is varied by varying the high pressure fuel injection within the cylinder after the port is closed. To control such adjustments over a wide range is difficult, costly, and has not been proven satisfactory.